The present invention relates generally to electromechanical pilot wire protective relays, and more specifically to direct transfer trip apparatus for shared use with an active pilot wire communications channel which couples signals between electromechanical units of a pilot wire current differential relay.
Conventional electromechanical pilot wire differential relays using a continuous metallic wire pair to complete the protective relay circuit have long been applied to the protection of short power lines of a power system network. Exemplary of this type of protective relay is the Westinghouse HCB and/or HCB-1 which are described in the Westinghouse Electric Corporation publication "Applied Protective Relaying" 1979, pp. 14-1 through 14-9. This type of relay scheme is coupled to the protected line much in the same manner as that shown in the block diagram schematic illustration of FIG. 1.
These pilot wire relays for the most part have provided adequate protection of their power lines through the years. However, there have been times when the metallic pilot wires have caused severe relay mis-operations, especially during a line fault condition. Some of the mis-operations have occurred due to a station ground mat rise resulting from the fault condition. Others have resulted from electromagnetic interference (EMI) being coupled to the pilot wires which may be run longitudinally along the path of the protected power lines. In addition, further complications come from the necessity to obtain a right of way for and/or the leasing of the pilot wires which transactions are becoming increasingly more costly and difficult to negotiate. It has become of paramount importance to all parties concerned to overcome these drawbacks.
Some relay manufactures have developed more modern and sophisticated current differential relay systems to meet the aforementioned need. One such system is the Westinghouse LCB current differential relay which is described in the Marketing Bulletin B-796 (May, 1983) issued by the Relay and Telecommunications Division of Coral Springs, Florida. The U.S. Pat. No. 4,275,429 entitled "Protective Relay Apparatus" issued to Larry L. Church and Shan C. Sun on June 23, 1981 and assigned to the same assignee as the instant application also discloses a similar type relay. While these relays offer a total answer to the aforementioned drawbacks for present and future protective relay needs, they do not offer a viable pilot wire replacement or retrofit for the thousands of existing and deployed electromechanical pilot wire relays other than for a total replacement which for the most part would be considered cost prohibitive. Thus, a problem remains for the already installed electromechanical pilot wire relays.
The disclosure which follows proposes an active pilot wire coupling scheme which is offered as a direct replacement for the continuous metallic pilot wire pair of the presently installed electromechanical current differential relays. It is believed that such relays, like the Westinghouse HCB/HBC-1 relay, for example, may be readily interfaced with the disclosed scheme without the need of any relay modification thereby preserving completely the performance characteristics of the current differential relay.
In addition, some direct transfer trip functions for the conventional electromechanical pilot wire relays are implemented with a dedicated communications channel and separate equipment from that of the pilot wire relays. Others require the application of a separate control signal, such as in the Westinghouse HCB, for example, which uses a DC potential applied directly to the pilot wire for direct transfer trip control. With the foregoing proposed active pilot wire coupling scheme, it is possible to share the communication of both the relay signalling and direct transfer trip keying over the same active coupling channel. However, in doing so, it is imperative to absolutely preclude the possiblity of an inadvertent breaker operation associated with operation of the current differential relay during the direct transfer trip keying period. The following disclosure additionally proposes direct transfer trip apparatus for use with the active pilot wire coupling channel which apparatus also satisfying the aforementioned requirement.